The typical brushless DC or three phase electric motor driven by a three phase bridge or inverter uses the motor windings to integrate the square wave output drive pulses from the bridge. However, timing and switching of power in an inverter to drive a motor generates significant electromagnetic interference (EMI). When the controller, and thereby the bridge or inverter driver circuit is remotely located from the motor load, a three conductor shielded cable is commonly employed as the interface between the driver and motor. This cable, in conjunction with the reactance of the motor and parasitic winding capacitance, creates high Q factor common mode resonances that result in high frequency damped sinusoidal ringing on the square wave bridge driver output pulse edges. This high frequency ringing increases electromagnetic emissions from the system and interferes with the ability to monitor and control the motor phase currents.
Snubber circuits are typically used to reduce the Q factor of the motor interface at the ringing frequency as a way to control electromagnetic emissions and improve the pulse shape of pulse output by the driver bridge. The snubbers could be grounded to the motor case, but this could increase radiated electromagnetic emission levels due to increased structure currents. Therefore what is needed is a way of reducing the Q factor of the motor interface without increasing structure currents and electromagnetic emissions.
In general, EMI noise can be divided into two major groups: differential mode (DM) noise and common-mode (CM) noise. DM noises are conducted between phases of the motor or inverter. CM noises are conducted together with all phases through the parasitic capacitance of the motor windings to structure ground. CM noises can be problematic for motor drives because CM noises increase the EMI in the motor drive and can damage the motor bearing and winding insulation. Unfortunately, in certain applications, solutions such as adding CM filters to attenuate CM noises are not viable due to the significant weight penalty of each CM filter.